This invention relates to an optical filter for the absorption of electromagnetic radiation. More particularly it relates to an optical filter for the absorption of electromagnetic radiation having a wavelength in the region of 532.times.10.sup.-9 m whilst allowing transmission of the remaining electromagnetic radiation.
In certain circumstances it is desirable to absorb electromagnetic radiation in a narrow wavelength band by means of an optical filter whilst allowing transmission of a substantial portion of the incident radiation, i.e. that at other wavelengths. In situations where laser radiation i.e. coherent radiation of a defined wavelength, is employed, such as certain medical and industrial applications, the beam of radiation may contact the eye or other device and cause damage thereto. In these situations absorption of electromagnetic radiation over the narrow wavelength band corresponding to the wavelength of the radiation emitted by the laser is necessary to protect the eye or optical device. Conventionally this has been achieved by way of an optical filter which whilst absorbing the narrow wavelength band corresponding to the wavelength of the laser, transmits the substantial portion of the incident radiation i.e. that at other wavelengths. Such optical filters are commonly manufactured by incorporating a compound which has particular absorption characteristics into a suitable host material, for example a polymeric matrix such as polycarbonate.
U.S. Pat. No. 4,622,174 discloses a transparent protective laser shield comprising a transparent host material and having at least one chromophore incorporated therein. The or each chromophore is selected from a porphyrin complex which has been modified by a metal, so providing metallo porphyrin complexes in the transparent material which absorb optical radiation of a pre-determined narrow wavelength band located within a selected broad wavelength band passed by the transparent host material. In particular, platinum octaethylporphyrin (PtOEP) is disclosed as being particularly suitable for absorption of neodymium YAG--doubled laser radiation at 532.times.10.sup.-9 m. Although PtOEP absorbs strongly at 532.times.10.sup.-9 m, its properties of photostability are very poor and unsuitable for a number of applications.
U.S. Pat. No. 4,657,345 discloses a similar shield in which the absorbers are diffused into a host material adjacent to its surface rather than being disseminated uniformly through it.
UK Patent Application No. 2225445 discloses a similar type of optical filter in which platinum deuteroporphyrin IX dimethyl ester is incorporated into the host as the absorbing material. This absorber has an absorption maxima at 533.times.10.sup.-9 m.
In each of the above-mentioned documents platinum is used to form the metallo porphyrin complex in the absorbing material and consequently the absorbing material is relatively expensive.